3. NON-GASEOUS X-RAY EMISSION

The strength of central X-ray emission should depend on the accretion
process and black-hole mass, coupled with the effects of geometry and
absorption. X-ray imaging has insufficient angular resolution to
separate such emission from beamed radiation associated with
an inner radio jet. However, in sources where an absorbed power-law
component is present, it is easily recognized if it is dominant,
and a heavy excess absorption helps to make spectral separation
possible even if much of the emission is from
surrounding hot gas. The best case is the hard X-ray detection of
emission from the core of Cygnus A
(Section 2.1), and absorbed power-law
components are
claimed in other radio galaxies with ROSAT, BeppoSAX and ASCA (e.g.
Allen & Fabian
1992,
Trussoni et
al. 1998,
Sambruna et
al. 1999).

Where the excess absorption is only modest, and this covers many of
the cases where the absorbed X-ray component is dominant, an
association with the central engine is questionable, and the absorbed
X-ray emission is most likely beamed emission associated with the
radio jet (Section 3.2). This is illustrated by
NGC 6251, where the X-ray absorption of ~
1021 cm-2 agrees both with that
inferred from reddening through the large-scale disk measured with HST
(Ferrarese &
Ford 1999)
and with an HI radio absorption-line measurement
(Worrall &
Birkinshaw 1999b),
and where the strength of X-ray relative to radio
emission in comparison with other radio galaxies argues independently
for a radio-related origin for the power-law X-ray emission
(Worrall &
Birkinshaw 1994).
The `puzzling' excess absorption seen in BLRGs
(Sambruna et
al. 1999)
might also be explained, at least in part, by cool gas
on larger scales than an inner torus absorbing jet-related
X-rays. This is consistent with the relatively strong X-ray emission
of BLRGs and the required orientation of their jets under Unification
models.

ROSAT pointed observations have shown that the central soft
X-radiation of low-power radio galaxies is almost certainly dominated
by nonthermal emission associated with the radio jet.
Canosa et al. (1999)
find that the core X-ray and radio emission are
well correlated in the B2 radio-galaxy sample
(Fig 5),
and a similar situation holds for the low-power 3CRR radio galaxies
(Hardcastle
& Worrall 1999a).
M 87 and Cen A are sufficiently close that
jet-related X-rays are resolved, and the fact that their X-ray to
radio ratio is similar to that for more distant unresolved X-ray cores
is further support for a jet-related origin of the core soft X-ray
emission in all such sources
(Worrall 1997).
Although in principle
such X-ray emission could be either synchrotron or inverse Compton in
origin, the relative proportions of radio, optical (HST) and X-ray
core emission, as compared with radio-selected BL Lac objects, argue
in favor of inverse Compton emission and predict a relatively flat
spectral index
(Hardcastle
& Worrall 1999b).
Flat-spectrum components
superimposed on thermal X-rays from hot gas are reported in the ASCA
spectra of several low-power radio galaxies, but are variously
interpreted as thermal emission associated with an advection dominated
accretion flow
(Allen et al. 1999)
and as higher than previously suggested (e.g.
(Fabbiano et al. 1989)
X-ray emission from stellar and post-stellar X-ray sources
(Matsumoto et
al. 1997).
Jet-related X-ray emission is also
likely to be a major contributor to the compact soft X-ray emission of
powerful radio galaxies
(Hardcastle
& Worrall 1999a)
see Fig 3 and, although in general
their greater distance
with respect to low-power sources leads to the expectation of a larger
contribution from extended gaseous emission within an unresolved X-ray
core.

Figure 5. Core X-ray vs. core radio
luminosity for the sample of
B2 low-power radio galaxies observed with ROSAT in pointed
observations, after separation of any extended X-ray emission. The
best-fit correlation taking into account non-detections (solid line)
excludes (based on astrophysical arguments) a starburst galaxy (cross)
and the broad-line radio galaxy 3C 382 (diamond) which fall in the
sample. Radio galaxies in previously known optical clusters are shown
as squashed squares, relic radio sources are open circles,
and other sample members are shown as stars. Figure from
Canosa et
al. (1999).

X-ray emission from compact radio hotspots has been detected in a
handful of sources, as summarized by
Hardcastle et
al. (1998a)
and
Harris (1998).
Such measurements are potentially of great
physical interest since the radio-emitting regions are normally well
localized and, if it can be shown that the X-rays are of inverse
Compton origin (e.g.
Harris et
al. 1994a),
the radio and X-ray
emission together probe the magnetic field strength and the balance
between particle and magnetic energy density. A similar probe on
larger scales is provided through inverse Compton scattering of cosmic
microwave background (CMB) photons on particles in the radio lobes, with
detections reported in a few sources
(Feigelson et
al. 1995,
Tsakiris et al. 1996,
Tashiro et al. 1998).
Brunetti et
al. (1999),
in discussing extended emission in 3C 219, have
emphasized the role of AGN photons in Compton up-scattering, but at
present sources with extended X-ray emission of gaseous origin
vastly outnumber those for which extended inverse Compton X-ray
emission is likely to have been detected.